Thermodynamic analysis of the coal-driven solar thermochemical cycle for hydrogen production

The two-step solar thermochemical cycle for hydrogen production has emerged as a promising technology for efficient solar energy utilization. It harnesses solar energy by employing reduction and oxidation processes within the cycle to continuously produce hydrogen. However, the existing challenges of high reaction temperatures and low oxygen partial pressures in the reduction process hinder the enhancement of solar energy conversion efficiency. In this study, we propose a coal-driven solar thermochemical cycle hydrogen production system, which utilizes the byproducts of the two-step conversion of coal to assist in the reduction of the oxygen carrier during the thermochemical cycle. This method can simultaneously reduce the reaction temperature and the oxygen partial pressure to improve the cycle efficiency. Through the coupling of solar energy and coal resources, the efficient conversion of coal is realized, and the capture and utilization of carbon dioxide in the conversion process of coal is realized through solar thermal reduction, which can achieve zero or even negative carbon emissions. The hydrogen production efficiency of the thermochemical cycle system with SnO₂ as the oxygen carrier can reach 52.1% without heat recovery, and the hydrogen production per unit mass of coal can reach 91.5 mol/kg. In the case of coal addition, our research has achieved carbon directional enrichment and obtained high hydrogen production efficiency, which provides valuable insights for the clean and efficient utilization of coal and the improvement of thermochemical cycle efficiency.